This invention relates generally to a white light illumination system, and specifically to a solid state lamp containing a semiconductor light emitting diode (xe2x80x9cLEDxe2x80x9d) or laser diode, a light scattering material, and a luminescent phosphor or dye material.
Light emitting diodes are semiconductor chips that are mounted in a package and emit radiation in response to an applied voltage or current. These LEDs are used in a number of commercial applications such as automotive, display, safety/emergency and directed area lighting. Recently, a white light emitting lamp was developed which includes a blue light emitting diode and a yellow emitting phosphor
As discussed in chapter 10.4 of xe2x80x9cThe Blue Laser Diodexe2x80x9d by S. Nakamura et al., pages 216-221 (Springer 1997), incorporated herein by reference, white light LEDs are fabricated by forming a ceramic phosphor layer on the output surface of a blue light emitting semiconductor LED. Conventionally, the blue LED is an InGaN single quantum well LED and the phosphor is a cerium doped yttrium aluminum garnet (xe2x80x9cYAG:Cexe2x80x9d), Y3Al5O12:Ce3+. The blue light emitted by the LED excites the phosphor, causing it to emit yellow light. The blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light. However, these white LED lamps suffer from the undesirable halo effect and the penumbra effect.
The halo effect occurs due to the separation of blue and yellow light. The LED emits blue light in a directional, anisotropic fashion. However, the phosphor emits yellow light isotropically (i.e., in all directions). Therefore, when the light output by the system is viewed straight on (i.e., along the LED emission direction), the light appears bluish-white. In contrast, when the light output is viewed at an angle to the LED emission direction, the light appears yellowish due to the predominance of the yellow phosphor emission. When the light output by such a system is directed onto a flat surface, it appears as a yellowish halo surrounding a bluish area.
The penumbra effect is similar to the halo effect, except that the halo effect is a color separation effect, while the penumbra effect is a non-uniform intensity effect. The penumbra effect causes the white LED lamp emission to appear brighter at the center than at the edges. As noted above, the LED emission is directional, while the phosphor emission is isotropic. Therefore, the white light emitted by the lamp appears brighter at the center, where the LED light is directly visible and where the LED light directly excites the phosphor. In contrast, the less bright edge of the white light emission is due to the stray and/or reflected LED light and the phosphor emission excited by such LED light. The halo and/or penumbra effects cause the white LED lamps to fail to meet applicable commercial quality standards required for illumination devices.
FIGS. 1 and 2 illustrate the halo effect and the penumbra effect. In FIG. 1, the white LED lamp 1 contains a blue emitting LED chip 3, a thick phosphor containing silicone layer 5 over the LED chip 3, and a reflector cup package 7 supporting the LED chip 3. The blue emission from the LED chip 3 is transmitted through the phosphor containing layer 5, and is mixed with the yellow emission from the phosphor in layer 5, which together appear as white light 10 to an observer. FIG. 2 shows a plan view of the lamp 1 emission. As can be seen in FIG. 2, the lamp 1 emission appears bluish in the center, and yellowish around the periphery thus illustrating the halo effect. Furthermore, the lamp 1 emission appears brighter at the center than at the periphery, thus illustrating the penumbra effect.
One conventional approach to decrease the halo effect is to incorporate a light scattering or diffusing material into the lamp. For example, U.S. Pat. No. 6,066,861, incorporated herein by reference, describes a white light emitting lamp 11 where the light scattering or diffusing particles 16 are incorporated into the same epoxy encapsulating material 15, which contains the YAG:Ce3+ phosphor. The epoxy 15 is located over the blue LED 13, as illustrated in FIG. 3. The LED chip 13 contains lead and wire electrodes 12 and a reflector cup package 17.
Alternatively, U.S. Pat. No. 6,069,440, incorporated herein by reference, discloses placing the light scattering material above the phosphor material, as illustrated in FIG. 4. In FIG. 4, the white light emitting lamp 21 contains a blue LED 23 located in a reflector cup 27 portion of one of the lead electrodes 22. The LED chip 23 is connected to wire and lead electrodes 22. An epoxy material 25 containing the YAG:Ce3+ phosphor is located directly above the blue LED 23. The light scattering or diffusing particles are incorporated into a molding layer 26 above the phosphor containing epoxy 25.
Therefore, in both of the above conventional lamps 11, 21, the light scattering material is positioned above or at the same level as the phosphor. The light scattering material scatters both the blue light transmitted through the phosphor and the yellow light emitted by the phosphor.
The present inventors have determined that, in the prior art LED lamp 11 of FIG. 3, the penumbra effect has its origins in the dissimilarity of the spatial extent and degree of collimation of the yellow and blue sources of light. The blue source is the LED (or laser diode) chip itself, and the light emitted from the chip is at least partially collimated by waveguide effects, especially in the lateral direction. The yellow source is the ensemble of phosphor particles, each of which emits yellow light in a random direction at intensity directly proportional to the intensity of blue xe2x80x9cpumpingxe2x80x9d light at the spatial position of the particle. To further complicate the situation, both blue and yellow light rays are elastically scattered by the phosphor particles. Finally, the amount of yellow light emitted by a given phosphor particle depends on its blue absorbance, which depends on its shape and size.
An ideal phosphor coating would contain a high concentration of phosphor particles in regions of high blue light intensity, and a lower concentration of phosphor particles in regions of lower blue light intensity, such that the spectrum of light emitted from rays in any direction and from any two points on the periphery of the lamp differ by only a scaling constant. In practice, however, this is difficult to implement when the blue and yellow source geometries are so dissimilar. The present invention is directed to overcoming or at least reducing the problems set forth above.
In accordance with one aspect of the present invention, there is provided a light emitting device, comprising a radiation source, a luminescent material, and a radiation scattering material located between the radiation source and the luminescent material.
In accordance with another aspect of the present invention, there is provided a white light emitting device, comprising a package containing a reflector cup, a light emitting diode in the reflector cup, radiation scattering particles in a packed layer or in carrier medium over the light emitting diode, and a phosphor or an organic dye which emits radiation having a second peak wavelength in response to incident light emitting diode radiation having a first peak wavelength, such that the device output appears white to an observer.
In accordance with another aspect of the present invention, there is provided a method of generating white light comprising supplying power to a light emitting diode, generating a directional blue light or ultraviolet radiation, passing the blue light or ultraviolet radiation through a light or radiation scattering material to diffuse the blue light or ultraviolet radiation in a plurality of directions providing the diffuse blue light or ultraviolet radiation onto a luminescent material, and generating white light.
In accordance with another aspect of the present invention, there is provided a light emitting device, comprising a radiation source, a discrete luminescent material layer which exhibits substantially no Mie scattering, and a discrete radiation scattering phosphor layer located between the radiation source and the luminescent material, which exhibits Mie scattering.
FIGS. 1, 3 and 4 are side cross sectional views of prior art white LED lamps.
FIG. 2 is a plan view of the emission of the lamp of FIG. 1.
FIG. 5 is a schematic illustration of the method of operation of a white LED lamp according to the preferred embodiment of the present invention.
FIG. 6 is a graph of relative light scattering power versus particle diameter.
FIG. 7 is a schematic side cross sectional view of a white LED lamp according to the preferred embodiment of the present invention.